The invention herein relates generally to cameras but particularly is concerned with a camera or a camera system which uses an electrophotographic film as the light responsive member upon which an image is to be recorded.
The electrophotographic film which is used in the camera of the invention is described in detail in copending application Ser. No. 378,180. It differs in many basic and important respects from the photosensitive film of the so-called silver halide type that is used today for relatively high speed photography. Such electrophotographic film, while generally operating on the basis of certain principles that it has in common with known electrostatic members also differs radically from such members in its composition, speed and characteristics.
The conventional photographic film has a relatively thick emulsion which is light sensitive. Black and white film uses a silver halide emulsion as mentioned above and colored film uses complex plural layers of light sensitive dyes in addition as photosensitive means. Such photographic films are highly light sensitive and hence must be manufactured in the dark or under special light conditions; cannot be stored or transported except in light-tight packages; must be introduced into a camera in darkness and maintained in darkness within the camera until use, and must be removed from the camera and processed in darkness or special light. Aside from these difficulties, conventional photographic film is sensitive to temperature and humidity; has limited shelf life; in most cases is grainy with the grain size increasing with film speed. Film speed as a general rule is controlled by the materials from which the photographic film is made and the methods of making the same such that the ambient light conditions existing when the film is used become a factor which the user must consider with some delicacy. The size of the aperture and speed of the shutter of the camera which carries the film must be adjusted for theoretically optimum results, which, notwithstanding automatic cameras, is not always truly optimum.
As an example of the problems mentioned, if a user assumes that a set of conditions will exist for outdoor photography that warrants the use of low speed film, as for example bright sunshine, he will install such film in his camera. Thereafter, should the ambient conditions change to, for example, heavy overcast, the film speed may be too slow for the types of scenes needed to be recorded, even at the largest aperture and slowest speeds of the camera. Even if the camera system permits of such size opening and speed capable of capturing the desired images, depth of focus will suffer because of the necessity of using large apertures. The user may be required to change film in the field and often in the middle of a roll.
While the above problems are inherently solved by the electrophotographic film of the copending application Ser. No. 378,180, the invention herein enhances the value of the said electrophotographic film by providing a vehicle to enable the best characteristics of the film to be utilized. The film sensitivity is adjusted in accordance with the ambient light conditions so that it is immaterial to a great extent what such light conditions may be. Further, in the conventional photographic film of the silver halide type the range of greys is substantially limited, but the camera of the invention which uses the electrophotographic film of the copending application Ser. No. 378,180 provides an almost unlimited range of grey tones with little regard to the ambient light conditions. The basic concepts of adjustment of sensitivity of the electrophotographic film used in the camera of the invention are disclosed in copending applications Ser. Nos. 389,124, and 389,149.
One of the most delicate and critical components of a modern camera or camera system intended to use light-sensitive photographic film is the shutter. The shutter serves two important purposes in the modern camera, the first of which is to keep the emulsion of the film in total darkness until it is used, the second of which is to time precisely the exposure of the film. The camera of the invention eliminates the shutter and much of the expensive, complex attendant mechanisms required to control the shutter.
The electrophotographic film of copending application Ser. No. 378,180 may be exposed to light at all times prior to its use so that no special precautions need be taken to keep it in darkness. When it is to be used, it can be exposed to a still scene without a shutter and charged while at the same time being exposed as will be explained. Even moving scenes could be "stopped" under certain circumstances, although at the present state of the art it would be preferred to use a shutter for moving scenes. The criterion of a shutterless camera, in any event, revolves around the inertness of the uncharged film when exposed to light.
The electrophotographic film used with the camera of the invention is not light sensitive in the sense that any permanent chemical change occurs when it is subjected to light. The photoconductive coating of the electrophotographic film is charged by being subjected to a relatively high potential corona and the impinging photons from the ambient light cause the flow of electrons from the surface of the coating and holes to the surface of the coating in proportion to the amount of incident light reaching each increment of the coating. The latent image is represented by an absence of electrons at the incremental locations which have received light and a presence of electrons at the incremental locations which have not received light from the scene to which the electrophotographic film is exposed. The incremental areas between absolute darkness and maximum light have variable degrees of electrons at or near the surface of the coating and represent grey tones of the scene.
The camera of the invention is in effect an image recording device which operates on the very basic of principles which are embodied in the well-known xerographic and electrofacsimile (electrofax) copying machines known at the present time. The essential differences between the camera of the invention and the image recording devices of the prior art lie in the simplicity and flexibility of the camera of the invention and its speed. While it is to some extent an important consideration that the electrophotographic film to be used with the camera of the invention provides a major basis for the advantages and versatility of the camera, the apparatus itself has inherent aspects which are of great benefit independently of the type of film used.
The xerographic and electrofax apparatus as known utilize fixed charging times, fixed lighting conditions and fixed exposures. The slow speed of the electrostatic members and their slow charge acceptance contribute also to the nature of the apparatus and complexity of known copying machines. Their use as cameras in the same sense as the conventional photographic cameras is in most cases impractical if not impossible. For example, it is believed to be outside of the realm of practicality to build a hand-held camera of a so-called pocket or portable size which utilizes the structures and principles of the modern xerographic or electrofax equipment; hence it is certainly not obvious to those in this field that such could be done. To build such a camera for still photography would seem to pose great problems, and to achieve the capture of moving images on electrostatic members clearly would exacerbate the problems. The invention teaches the solution of the problems to achieve a practical still camera and points the way to the achievement of a camera for moving scenes.
Further to emphasize the achievement of the invention, at least some of disadvantages of the conventional photographic camera have been obviated by utilizing a camera without a shutter. Thus, the disadvantages of both the conventional camera and the electrostatic apparatus of the prior art are eliminated and the result is a remarkable camera that has great utility.
In the known electrostatic apparatuses the electrostatic member, typically a coating of amorphous selenium on metal or a coating of a zinc oxide-resin mixture on a sheet of conductive paper, must first be charged in darkness for a substantial period of time. A charging period of one second or thereabouts is common. The charging in darkness is needed to preserve as much charge as possible, and the time of charging is long because these members do not accept charge at a fast rate. The next step involved is the exposure to a scene, and this is of necessity done with brilliant light projecting an image onto the charged surface of the electrostatic member. As stated, the time of exposure is fixed and the amount of light is fixed since the process takes place on a light table and the image is projected onto the surface of the member by various systems of mirrors, lenses, etc. The exposure time is relatively long, typically a second or large fraction thereof because the light discharge of these members is slow and the exposure time must be long to achieve good images. At that, the images which are produced are photographically of poor quality, even with the best of the known copying apparatus.
The camera of the invention operates in a simple manner with the electrophotographic film mentioned above, or any other film which has speed and charge acceptance that is comparable. The various factors involved include charge time and hence surface charge potential, size of aperture of the projecting system, time of toner application and the bias used during toning. It will be noted that time of exposure is not mentioned, it being the same as the time of charging as will be explained. Means for adjusting several factors may be provided in a practical camera, but the simplest camera according to the invention provides only for the adjustment of the time of charge.